This invention relates to an automatic developing bias control device for use with an electrophotographic copying machine and more particularly it relates to an automatic developing bias control device for an electrophotographic copying machine in which a document image is led onto a photoreceptor through a lens to form an electrostatic latent image on the photoreceptor and the electrostatic latent image is visualized into a toner image by a developing device, said automatic developing bias control device operating on the principle of detecting the density of the document image, and setting the developing bias potential which is to be present during development of said document image so that said potential corresponds to said output detection signal.
Conventionally, because of the need to provide optimum copy image quality with respect to a variety of documents, the operator visually judges the print density of the document and manipulates the density control knob installed in the operating section so as to change the amount of exposure or the bias potential of the developing device to thereby adjust the print density of the copy.
In the aforesaid method relying on the operator's visual judgment, however, the operating characteristics are unsatisfactory or visual judgment varies from one operator to another, resulting in a problem that copies having optimum image quality cannot always be produced.
Therefore, in recent years, there has been adopted a system in which the intensity of light reflected from the original is detected and the resulting output detection signal is used to control the amount of exposure of the original or a second system in which the bias potential of the developing device is automatically controlled.
In the former system, which is disclosed for example in U.S. Pat. Nos. 3,279,312, 3,609,038, and 3,914,049, since the response of the light source is poor, it is impossible to control the amount of exposure in an instantaneous manner based on image information obtained by exposing the original. Further, it may be contemplated to control the amount of light reflected from the original by means of a shutter mechanism U.S. Pat. No. 3,438,704), but in this case also there is the same drawback as that described above since the response of the shutter mechanism, which is mechanically driven, is poor.
On the other hand, the latter system is superior, involving no problems inherent in the preceding systems, because of its high response with which the bias potential of the developing device is changed. U.S. Pat. No. 4,304,486 discloses a copying machine based on such system, wherein a light detector receives the non-condensed portion of the reflected light to detect the light intensity, and a bias potential determined by the detected intensity is applied to the developing device. With such a copying machine, the intensity of reflected light from each document is detected to control the bias potential of the developing device, so that copies having optimum copy image quality can be obtained.
However, this technique is applicable only when the document image information does not vary with the exposure travel direction (the direction in which the document support platen or the optical system travels for slit exposure purposes) so that the document can be judged te be light or dark as a whole. Even if it is applied to a document having such variation dependent on the exposure travel direction, it is impossible to obtain a copy having optimum image quality. That is, where it is applied to a document having such variation, there has been a problem that a copy having erroneously controlled image quality is produced, such as a copy in which some portions have optimum copy image quality and others do not, or a copy in which all portions deviate from the level of optimum copy image quality.
An object of this invention is to provide a novel automatic developing bias control device for an electrophotographic copying machine which is designed to detect image information from every exposed portion of a document during exposure travel and to set the bias potential of the developing device, which is present at the instant of developing each of said image information portions, at the proper value so that it corresponds to the output detection signal.
This invention comprises scanning and exposing means for scanning and exposing a document, detecting means for detecting the amount of light reflected from the document, delaying means for delaying the output detection signal for a period of time dependent on the time taken for the electrostatic latent image of the document to travel from the position of formation of the latent image by the scanning and exposing operation to the position of image visualization by the developing device, and control means for changing the bias potential of the developing device so that it corresponds to the output from the delaying circuit.
As for the detecting means, at least one such means is provided inside or outside the optical path, it being only necessary that the means be capable of detecting the amount of reflected light by the reflected light itself or by disturbance light. It is disposed at a position remote from the document image forming position, for example, in the vicinity of the lens or upstream or downstream of the lens, so as not to produce adverse effects on the formation of images on the photoreceptor surface.
The control means includes means for changing the bias potential continuously with respect to the delayed output and also means for changing it stepwise.